(1) Field of the Invention
The present invention relates to a constant flow rate expansion valve, and more particularly to a constant flow rate expansion valve for use in a refrigeration cycle for an automotive air conditioner, which adiabatically expands high-temperature, high-pressure refrigerant to thereby change the high-temperature, high-pressure refrigerant into low-temperature, low-pressure refrigerant to deliver the refrigerant to an evaporator at a constant flow rate.
(2) Description of the Related Art
In general, a refrigeration cycle for an automotive air conditioner comprises a compressor, a condenser or a gas cooler, an expansion valve, an evaporator, and an accumulator. Out of these component elements, the compressor and the expansion valve control the flow rate of refrigerant. In a variable displacement compressor, to control the flow rate of refrigerant, an electronic control valve is sometimes used which performs differential pressure control such that the difference between discharge pressure and suction pressure of the compressor becomes constant irrespective of the flow rate of refrigerant. In this case, it is recommended that for an expansion valve for electronic control, a constant flow rate expansion valve that controls the flow rate of refrigerant such that it becomes constant irrespective of the differential pressure across the expansion valve is employed so as to avoid contention between the differential pressure control carried out by the electronic control valve of the variable displacement compressor, and the control by the expansion valve.
A known constant flow rate expansion valve is described e.g. in Japanese Unexamined Patent Publication (Kokai) No. 2001-153495 (FIG. 1). This expansion valve is constructed by combining a solenoid that is capable of changing a flow passage cross-sectional area of a refrigerant passage at a part where refrigerant is adiabatically expanded, and a constant differential pressure valve that keeps constant the differential pressure across the refrigerant passage of which the flow passage cross-sectional area is set by the solenoid. Therefore, the constant differential pressure valve controls the differential pressure across the refrigerant passage of which the flow passage cross-sectional area is determined by the solenoid such that the differential pressure is held constant, and therefore the flow rate of refrigerant flowing through the refrigerant passage is controlled to be constant. The flow rate can be freely set by the value of electric current supplied to the solenoid.
Further, the differential pressure valve includes a piston that senses the differential pressure across the refrigerant passage by opposite ends thereof, and operates in the valve-opening or valve-closing direction according to the differential pressure, and a constant flow rate expansion valve is also known which has improved degradation of characteristics due to small leakage of refrigerant via a sliding portion of the piston (see e.g. Japanese Unexamined Patent Publication (Kokai) No. 2004-53192 (FIG. 1)).
According to this constant flow rate expansion valve, a main valve, and a restriction device interlocked with a solenoid are arranged in series in the direction of flow of refrigerant, and in parallel therewith, a piston for actuating a main valve element, having a fixed orifice, a pressure chamber, and a pilot valve forming a variable orifice together with the solenoid are arranged in series. As a result, since the sliding portion of the piston is connected to the pressure chamber, refrigerant that leaks from the sliding portion leaks into the pressure chamber, and further the refrigerant having leaked is controlled by the pilot valve together with refrigerant discharged from the pressure chamber, whereby it is possible to substantially eliminate the influence of leakage of refrigerant from the sliding portion of the piston.
In the constant flow rate expansion valve constructed as described above, when the solenoid is controlling the pilot valve to a valve lift dependent on the value of electric current supplied thereto, the pressure chamber is under pressure corresponding to the size of the variable orifice formed by the pilot valve, and hence the piston controls the main valve to a valve lift dependent on the differential pressure between the pressure in the pressure chamber and pressure in an inlet port so that refrigerant flows through the main valve and the restriction device at a predetermined flow rate. Now, when the flow rate of refrigerant increases, the differential pressure across the restriction device interlocked with the solenoid increases, and therefore the differential pressure controls the pilot valve in a direction of decreasing the variable orifice. The pressure in the pressure chamber on the upstream side increases in proportion to decrease in the variable orifice, and the piston operates to reduce the flow rate of refrigerant by controlling the main valve in the valve-closing direction. Inversely, when the flow rate of refrigerant decreases, the main valve is controlled in the valve-opening direction for increasing the flow rate of refrigerant. As a result, the flow rate of refrigerant flowing through the constant flow rate expansion valve is controlled to a constant flow rate determined by the solenoid.
In the conventional constant flow rate expansion valve which is configured to control pressure in the pressure chamber by the pilot valve disposed on the outlet port side, the valve element of the pilot valve receives the differential pressure between outlet pressure and the pressure in the pressure chamber in the valve-opening direction. However, the pressure in the pressure chamber is close to inlet pressure, and hence the differential pressure applied to the valve element of the pilot valve is very large. This differential pressure acts as a disturbance for the pilot valve which operates by sensing the flow rate of refrigerant flowing through the restriction device, and hence it is desirable to eliminate the differential pressure. In this case, it is contemplated that a mechanism for canceling the differential pressure is added to the pilot valve. However, this makes the construction of the constant flow rate expansion valve very complicated, and therefore the pressure-receiving area of the variable orifice is made smaller to thereby lessen the degree of influence of the differential pressure on the pilot valve. However, there is a limit in decreasing the pressure-receiving area, and the degree of influence of the differential pressure in the valve-opening direction is large against a force which the pilot valve causes to act on the valve element thereof in the valve-closing direction, by sensing the flow rate of refrigerant, which makes the differential pressure unnegligible. This causes the problem that the pilot valve tends to act as if it were a constant differential pressure valve.